The present invention relates generally to methods for treating a host by implanting genetically unrelated cells in the host. More particularly, the present invention provides methods of treating a host by implantation of immortalized human fetal neuro-derived cells.
Organ transplantation has become a successful and widely practiced means of treating a variety of diseases. Cardiac, renal, and even liver transplants are almost routine in many medical centers. Unfortunately, disorders of many organs are not amenable to treatment with whole organ transplants. For example, lesions of the central nervous system may not be treated by whole organ transplants to replace damaged tissue.
Because replacement of injured tissue by whole organ transplant therapy is not possible for many diseases, or even for all patients having appropriate diseases, attempts have been made to develop methods of transplanting cells. Sun et al., Biomat., Art. Cells, Art. Org., 15:483-496 (1987). Parenchymal lesions which result in a deficiency of a biologically active compound may be treated by transplanting isolated cells or cell clusters that secrete the biologically active compound. For example, diabetic animals have been successfully treated by implantation of islets of Langerhans separated from donor pancreases. Noel et al., Metabolism, 31:184 (1982).
Cell transplant therapy is particularly appealing for treatment of neurological diseases. Solid tissue transplantation is especially inappropriate for neurological diseases for several reasons. Open surgical exposure of the brain, as required for solid tissue transplantation, can cause irreparable damage to nervous system pathways resulting in clinical neurological deficits. Also, neurological function often depends on complex intercellular connections which can not be surgically established. Further, cells of the central nervous system are exquisitely sensitive to anoxia and nutrient deprivation. Rapid vascularization of solid tissue transplants is critical as cells in the interior of solid tissue transplants often lack sufficient perfusion to maintain viability. Stenevi et al., Brain Res., 114:1-20 (1976).
One common neurological syndrome, Parkinsonism has been the object of attempts at cell transplant therapy. Bjorklund et al., Brain Res., 177:555-560 (1979); Lindvall et al., Science, 247:574-577 (1990); Freed, Restor. Neurol. Neurosci., 3:109-134 (1991). Parkinsonism is caused by a loss of dopamine-producing neurons in the substantia nigra of the basal ganglia. Burns et al., N. Engl. J. Med., 312:1418-1421 (1985); Wolff et al., Neurobiology, 86:9011-9014 (1989). Parkinson's disease, a disease of unknown etiology which is characterized by the clinical manifestations of Parkinsonism, is caused idiopathic destruction of these dopamine-producing neurons. Parkinsonism may be caused by a variety of drugs, e.g., antipsychotic agents, or chemical agents, e.g., 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Burns et al., Proc. Natl. Acad. Sci. USA, 80:4546-4550 (1983) and Bankiewicz et al., Life Sci., 39:7-16 (1986).
Attempts have been made to reverse the clinical manifestations of experimentally-induced Parkinsonism by transplanting dopaminergic cells into the striatum of affected animals. Genetically modified fibroblasts (transfected with DNA encoding tyrosine hydroxylase) have been successfully transplanted into animals having lesions of dopaminergic pathways. Motor function and behavior of the animals improved following implantation of the dopamine producing fibroblasts. Wolff et al., Proc. Natl. Acad. Sci. USA, 86:9011-9014 (1989); Fisher et al., Neuron, 6:371-380 (1991). Graft survival may be enhanced, and hence clinical improvement prolonged, by transplantation of fetal tissue, as compared to cells obtained following birth. Gage and Fisher, Neuron, 6:1-12 (1991). Fresh fetal dopaminergic neurons have been transplanted into the caudate nucleus of monkeys following chemical injury to the nigrostriatal dopamine system. Following transplantation, the injury-induced behavioral deficits improved. Bankiewicz et al., J. Neurosurg., 72:231-244 (1990) and Taylor et al., Prog. Brain Res., 82:543-559 (1990).
Humans suffering from Parkinsonism have been treated by striatal implantation of dopaminergic neurons. Lindvall et al., Arch. Neurol., 46:615-631 (1989); Widner et al., New Engl. J. Med., 327:1556-1563 (1992). The transplanted cells were obtained from abortions. Prior to the abortions, the women were screened for antibodies to several disease causing viruses. Following surgery, the treated patients exhibited improvement of neurological function. The patients required maintenance immunosuppressive therapy, however.
Recent investigations indicate that trophic factors released from support cells of the central nervous system (e.g., astrocytes and oligodendrocytes) are critical to survival of neurons in cell culture. O'Malley et al., Exp. Neurol., 112:40-48 (1991). Implanted fibroblasts that were genetically altered to express nerve growth factor have been shown to enhance survival of cholinergic neurons of the basal forebrain following injury to the fimbria-fornix which causes demise of acetylcholine neurons in the basal forebrain as seen in Alzheimer's disease. Rosenberg et al., Science, 242:1575-1577 (1988).
While previous attempts at cell transplant therapy for neurological disorders have provided encouraging results, several significant problems remain. The supply of fetal tissue for cellular transplants is quite limited. To ensure maximum viability, the fetal cells must be freshly harvested prior to transplantation. This requires coordinating the implantation procedure with elective abortions. Even then, fetal tissue has not been widely available in the United States. Also, the gestational age of the fetus from which cells are obtained influences graft survival. Gage and Fisher, supra. Obtaining fetal tissue of only certain gestational ages adds additional limitations to the availability of fetal cells for transplant. Further, ethical considerations make some potential transplant recipients reluctant to undergo the procedure when fresh fetal cells are implanted.
Because the fetal tissue is obtained from fresh abortuses, a significant risk of infectious contamination exists. Although women undergoing abortions which will supply fetal tissue are screened for a variety of infections, some infections, e.g. HIV, may not be clinically detectable and thus, not identified during the screening process. Therefore, if widely practiced, transplants of fresh fetal cells would likely cause many infectious sequelae.
Use of immortalized cell lines could overcome many of these difficulties of availability and infection only one immortalized human fetal neuro-derived cell line has been reported, however. Major et al., Proc. Natl. Acad. Sci. USA, 82:1257-1262 (1985) and U.S. Pat. No. 4,707,448. Further, immortalized cell lines, by their very nature, are predisposed to causing tumor formation following in vivo transplantation. Therefore, therapeutic intracerebral transplantations of immortalized cells carry a high risk of causing intracranial tumors, and even tumors having a benign histology may carry a poor prognosis when present within the calvarium.
Transplants of genetically unrelated cells carry the risk of immunological graft rejection and intracerebral inflammation. Widner and Brundin, Brain Res. Rev., 13:287-324 (1988). All transplants of genetically unrelated cells carry this risk. Therefore, patients treated by intracerebral cell transplant have required long-term maintenance immunosuppression which, even in the absence of transplanted immortalized cells, carries a high risk of infectious and malignant complications. The transplantation of immortalized cells only magnifies the risk of these complications.
What is urgently needed in the art are methods of therapeutically implanting immortalized human fetal neuro-derived cells. Ideally, the methods would not result in tumor formation or elicit intense inflammation following transplantation. Desirably, the methods could employ cells derived from cell lines so that the risk of infectious contamination and limited cellular availability would be minimized. Quite surprisingly, the present invention fulfills these and other related needs.